In nuclear medicine, quality control generally refers to those events by which the quality of a single component of the nuclear medicine procedure is governed. This is distinguished from quality assurance, which refers to a summation of all the functions by which imaging information is obtained from a procedure. Quality control is required within each are of the procedure to monitor the various parameters which can be defined and measured in order to guarantee the quality of the entire nuclear medicine process. Quality control in the operation of any nuclear medicine imaging system serves to ensure that the image represents the actual variation in radionuclide distribution within the patient and not variations in instrument performance. Therefore, those parameters important to the performance of imaging systems and associated instrumentation must first be evaluated and then measures routinely taken to ensure that these devices perform within the limits defined in the evaluation. Imaging instrument quality control is based on the principal that the image must represent an actual variation in the radiopharmaceutical distribution within the patient or the organ of interest and not an artifact introduced by the instrument or radioactive sources. The interpretation of static images and dynamic function studies is critically dependent upon the quality of the images. This dependence exists because significant changes in the condition of the patient may be reflected in subtle density changes in the image and could be confused with artifacts. Sub-optimal equipment performance or techniques may cause these changes to fall below the threshold of detectability and pass un-noticed by the physcian. For this reason, it is essential that nuclear medicine imaging equipment and procedures be maintained at an optimal level.
The three (3) major parameters which characterize gamma camera performance are spatial resolution, spatial distortion and field uniformity. Secondary parameters include temporal resolution, energy resolution and system sensitivity. Spatial resolution refers to the ability of an imaging system to produce two separate parallel line sources of radioactivity as separate entities. The smaller the distance between the two sources that can be resolved, the better is the spatial resolution of the system. Spatial resolution is also referred to as the system resolution or simply the resolution of an imaging instrument and is customarily represented by the Full Width At Half Maximum (FWHM) of the Line Spread Function (LSF). In general, there are two (2) methods for measuring resolution. The first is a direct method and determines the FWHM or more completely, the Modulation Transfer Function (MTF) from a plot of the line spread function (LSF). The plot is obtained utilizing a computer interfaced with the gamma camera system. This method, although quantitative and accurate, is not practical because it is complicated, time-consuming and specialized personnel are required to perform it. A second method for measuring resolution, with which the present invention is involved, is based on the principle that two adjacent parallel line sources which are spaced just one FWHM apart are imaged as a barely distinguishable double line. Therefore, the actual separation of the sources is equal to one FWHM if they can barely be distinguished. The prior art contains numerous transmission phantoms with a variety of line spacings for resolution measurements as a function of distance from the face of the camera collimator. These phantoms generally take the form of lead bars embedded in a plastic fixture. In one prior art phantom, the lead bars are grouped in sets of various widths and spacings. In another prior art phantom, a single array of lead bars is provided wherein the widths of all bars and spacings therebetween are equal and cover the full field of view of the scintallation camera. Still another prior art phantom includes four (4) sets of lead bars, each rotated 90.degree. in orientation with respect to the adjacent set. The spacings between are equal to the bar widths within a given quadrant. Each quadrant has different bar widths.
Spatial distortion refers to the ability of an imaging instrument to accurately reproduce planar rays of linear radioactive sources in a manner which preserves all the spatial and geometric relationships of the array. Lead-bar phantoms are also employed in the prior art to determine deviation of the image from a straight line.
Uniformity refers to the ability of a scintigraphic instrument to reproduce with fidelity an image of a uniformly distributed radioactive source, and has been recognized as the most important parameter to be monitored to ensure optimum gamma camera performance. Nonuniformities appear to be dependent upon spatial distortions, variations in the energy window between photomultipliers and variations in efficiency and resolution as a function of position. Lead-bar phantoms have not proven to be valuable in measuring uniformity.
In addition to the lead-bar phantoms described above, the prior art also includes the phantoms consisting of lead plates with holes drilled or punched therethrough in a suitable array. In one such phantom, there are a series of 6 pie-sliced shaped sectors of holes, each of the sectors containing holes of different spacing and size. Another prior art hole-type phantom provides a uniform hole pattern and spacing designed to cover the entire field of view of the camera.
None of the above-described phantoms permits simultaneous observation of resolution, uniformity and spatial distortion from a single transmission of the test pattern.